This question was asked yesterday or the day before...search around, there are some good answers in there (not to mention the many, many posts about this over the years). Also 'What should I charge' is one of the most popular questions on any photo related forum...so Google should give you plenty of reading, provided you search it thoroughly.
To really answer this for yourself, you need to figure out your business plan. As part of that, you should decide how you want to structure the way you charge for what you do. For example, you might charge $500 for a sitting fee, and then not need a whole lot of markup on your print sales. On the other hand, you might charge $20 for a sitting fee....and then need an 800% market from your 'cost of goods sold'.
Either way, you should really try to figure out all of your costs...not just the cost of the prints (goods sold), but how much did your gear cost? What about computer equipment that you used? Did you drive to the location or do you have a studio space (in home or not)? What about photographic education...including the time you spend practicing etc? Marketing? Don't forget taxes, business license, insurance etc.
These are things that need to be covered if you want a sustainable business. Not to mention making money for yourself.
Of course, it's unlikely that you'll be able to cover those costs with only 3-4 sessions a month (although if you could, that would be fantastic). So in your business plan, you should decide how many sessions you can do, how many you want to do and how many you'll need to do, in order to reach your goals.
All of this should help you to figure out where your pricing should probably fall.
One way to get this started, or to help find a starting point...would be to search around your area and see what other photographers are charging.